1. Field of the Invention The invention relates to optical disk players having removable disks and particularly for controlling electronic circuit operation with respect to the rotational position of the disk.
2. Discussion of the Prior Art Both magnetic and optical disk recorders are also referred to as direct access storage devices (DASD). These devices characteristically have either one or two data recording formats which determine the addressability of data stored on the respective disks. One type is a so-called fixed-block architecture (FBA) and the other is count-key data (CKD). FBA track formatting is described by Bohl, supra, beginning at page 82. The present invention is more applicable to the CKD format architecture.
CKD architecture is explained by Bohl, supra, beginning on page 72. CKD architecture is characterized by a single index mark, either embossed or permanently recorded, in each track of all of the tracks on the disk. No sector marks or other rotational position-indicating indicia are needed on the record disk. On those DASDs, such as the IBM 3380 Magnetic DASD, have record disks mounted on a single spindle for co-rotation along with a so-called servo disk. The servo disk has a precise fixed relationship to all of the record disks on the spindle and contains radial (tracking) positional information as well as rotational position information. However, when a single optical disk is removably mounted on a DASD, then the luxury of a servo surface is either very expensive, i.e. takes up one-half of the storage capacity of the disk, or is not available. It is desired to avoid a servo disk in this situation. The present invention is directed to such a system, employing a removable disk without rotational position-indicating indicia other than an index point. This arrangement maximizes the data storage capacity usage of the record disk.
When record disks store data at a high density, then rotational position must be precisely defined. Resynchronizing and recalibrating rotational position once per rotation of the disk at an index point is unsatisfactory as most oscillators will drift in phase and hence give erroneous rotational position indications. Another factor requiring precise rotational position indication is defect avoidance. In the IBM 3344 and 3350 disk storage units, provision is made for skipping over a surface defect on a track while still using the rest of the track for data. See page 30 of Bohl, supra.
In the home address area (HA) of each CKD formatted high-density track, a skip displacement from index is stored. In the 3344 or 3350 the servo surface provides for counting rotational position in fixed synchronism with all of the record disks on the common spindle. Since CKD architecture does not provide for rotational position indication on the disk surface and precision in rotational position detection is required, a suitable rotational position control is required. Note that on fixed-block architecture, as described on page 30 of Bohl, supra, a sector that has a defect in it is termed a defective sector, and a spare sector on the same track is used to replace the storage capacity of the defective sector.